11/4: Hydrologic Cycle and Groundwater
Where is Earth’s water?
Reservoir: storage place for water
o 96% in the oceans
o 3% in glaciers and polar ice
o 1% underground water (drinking water)
o <1% lakes/rivers, atmosphere, biosphere
Hydrologic Cycle
Models the movement of water from one reservoir to another (means and amount)
Warm air can hold more water vapors than cold air
Feiwjiwjoiji
Orogenic Rain Shadow
The deserts on the back side of a mountains
1. Prevailing winds carry warm air over oceans where it gathers moisture as water vapor
2. (look at slides for rest of steps)
3
4
5
Example: California
Precipitation: water that comes out of the sky (rain, snow, etc.)
Runoff: amount of water that travels along the surface
o Half the global runoff is carried in the 70 largest rivers – half of that is carried by the
Amazon alone
o Surface storage (reservoirs): lakes, wetlakes
Groundwater
Water table: the boundary between the unsaturated zone and the saturated zone
o Can move on a daily basis up and down
Groundwater is water in the saturated zone
Saturated zone: water fills all pore spaces
Unsaturated zone: water and air both occupy pore spaces
Porosity: percent rock volume that is pores (spaces); defines the amount of water a volume of
rock can hold
o Porosity increases with increased sorting
Poorly sorted material has a mix of big and small material to fill in the gaps
Example: Non-cemented sandstone vs cemented sandstone
Permeability: interconnectedness of pores
o Decreases with decreasing grain size, porosity, and sorting
o Direct relationship with porosity????
Infiltration
o Surface water infiltrates into the unsaturated zone by gravity and capillary forces, and
moves into the saturated zone
o Groundwater moves by gravity and pressure difference along a head gradient
o Groundwater moves from high head (high pressure) to low head (low pressure)
o Not always downhill
Recharge: addition of water that causes the water table to rise
Cone of depression: water is pumped faster than it can recharge
o Increased evaporation or withdrawal by pumping lowers the water table
o Salt water is denser than fresh water
o Too much pumping, and cities near the coast may find that their water supply turns
brackish as salt water is pulled into the discharge area
Aquifer: any permeable, saturated layer that can transport water
Aquitard: an impermeable layer that prohibits flow of water
Unconfined aquifer: upper boundary defined by the water table
Confined aquifer: upper boundary is an aquitard
Spring: a groundwater discharge feature that forms when the land surface intersects with the
water table
Artesian well: when the water pressure is so high at depth, the water shoots straight out of the
ground
Karst: general term referring to caves
When pumping exceeds recharge, the water is being mined, and the resource will eventually
deplete
o This is true for the US today
